For electronic applications with loads above 75 Watts a PFC (power factor correction) stage is mandatory. For electronic applications in some other fields, like e.g. in the field of lighting technology this is already the case at smaller loads. Power factor correction means that the wave shape of the current drawn by a load from the mains has a substantially sinusoidal form with a very small or no phase shift at all with respect to the wave shape of the mains voltage. The harmonic content is also required to stay below a certain threshold.
Common applications with PFC functionality mostly include a diode-based full wave rectifier which, for example in the case of a boost converter, supplies current to an output capacitor via an inductor and a further diode. A switch coupled between the inductor and the diode is employed in order to keep the drawn current in phase with the mains voltage. One of the disadvantages of such systems is that the relatively large output capacitor is charged via three rectifying elements during operation which leads to substantial losses. Until its voltage reaches the mains peak voltage, a precharging takes place and this precharging process entails large pulse currents which also flows at least through the rectifying diodes leading to high current stress to the system.
Alternative concepts aiming at reducing bridge rectifier losses rely on costly SiC and/or GaN switches or replace rectifiers with active rectifying schemes. However, those approaches lead to high costs which render them rather unattractive for circuit manufacturers, even though the efficiency of such circuits in relation to the reduction of losses may be improved.